Motor-generator/alternator sets have become useful in supplying electrical power requirements for computer applications because the power supplied directly from electric utilities contains far too much electrical noise which can easily produce computing errors. A motor-generator/alternator set provides the necessary buffered or isolated power protected system for these applications.
There are two popular types of motors being used to drive A.C. generators/alternators, namely, synchronous motors and induction motors. Synchronous motors always run at rated speed regardless of voltage changes. They would clearly be the choice for computer or data processing applications except for one major fault. Synchronous motors are hard to start.
This deficiency or shortcoming in synchronous motors is particularly troublesome in computer applications because the motor must restart after every momentary outage of electrical power. Heretofore, the solution has been to utilize costly, complicated, failure prone, solid state control packages to get the synchronous motor back on line instantly. Regardless of which control package is used, the users have experienced many more failures in the backup system than in the motor itself.
Induction motors, on the other hand, are easy to start. They restart on line after an outage without complicated control circuits thereby making them a clear choice for computer applications.
However, induction motors have an inherent shortcoming which heretofore has not been addressed for computer and data processing applications. Induction motors will not maintain their frequency over a broad voltage and load range by themselves. Computer manufacturers specify an input frequency range from 59.5 Hz to 60.5 Hz. This specification is important for memory units which are very sensitive to frequency. One other important specification established by the National Electrical Manufacturers Association (NEMA) is that voltage variation for motor input shall not exceed .+-.10% from a rated voltage of 460 volts even though most computer facilities are wired for 480 volts. NEMA says that motors should be able to run at full load indefinitely with a voltage input within .+-.10% of rated voltage.
However, induction motors have still another undesirable characteristic which tends to discourage their use in computer related applications; i.e., induction motors possess slip. "Slip" may be defined as the difference between synchronous speed (e.g. 1800 rpm) and the actual output speed in revolutions per minute. It is common to see industrial grade induction motors with slip ratings between 1% and 3%. This means that their output speed will be 1% to 3% less than synchronous speed. High cost, limited slip motors are readily available that reduce slip to as little as 1/2% but more commonly to around 0.7%. However, a slip of 1/2% yields an output of 59.7 hz. which is barely above the computer manufacturers' 59.5 hz. minimum requirement. If the voltage falls or the load increases, these induction motors no longer would produce acceptable computer power.